Heat Curing and Delayed Ettringite Formation
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M.C. LEWIS*, KAREN L. SCRIVENER* AND S. KELHAM** *Imperial College of Science, Technology and Medicine, London, U.K **Blue Circle Industries PLC, Kent, U.K
ABSTRACT This paper reports some preliminary results from a study of the effect of elevated temperature curing on mortars and the phenomenon of delayed ettringite formation (DEF). Mortars made from cements with sulphate levels of 3%, 4%, and 5% and with 5% sulphate and added alkali were cured at 20 and 900 C and subsequently stored in water. Expansion measurements showed a pessimum effect with increasing S03 content. Mortars which expanded showed a corresponding decrease in strength. X-ray diffraction (XRD) studies indicated that no ettringite is present after heat treatment but re-forms over time within the material. However, the ultimate levels of ettringite reached do not correspond to the magnitude of expansion observed. X-ray microanalysis shows that immediately after the heat treatment the aluminate species and most of the sulphate species are incorporated within the C-S-H gel. The concentrations of these species decrease during expansion, such that at the end of expansion the amounts remaining correspond to the presence of AFm phase mixed with C-S-H.
INTRODUCTION The heat curing of concrete is a widely used industrial techniques for the production of concrete structural members. This accelerates the rate of strength development, allowing the components to be demoulded and put into service more quickly. In recent years the deterioration of a number of structures which have undergone some form of elevated temperature curing and subsequent exposure to moisture has been observed. The damage is characterised by substantial surface cracking and erosion of the corners and edges. Internal examination of the material shows a heavily cracked paste with ettringite deposited in cracks and pores and in uniform rims around the aggregate particles. Damaged structures all seem to have undergone long term exposure to moisture and the macroscopic signs of deterioration become apparent after a number of years in service. Consequently a number of investigations have been carried out on the effect of heat treatment on cement pastes, mortars and concretes and on the subsequent formation of ettringite (e.g. refs 1-6). It is now accepted that if hydrating Portland cement is heated above a certain temperature, ettringite no longer forms and some or all of that which is formed prior to heat treatment is destroyed. If the material is subsequently kept in water or saturated air at ordinary temperature, ettringite is again formed. This ettringite subsequently recrystallizes around the aggregate particles as uniform rims as well as infilling voids and pores. In parallel with the reformation of ettringite, a general expansion of the materials is observed. This phenomena is generally referred to as delayed ettringite formation (DEF). Despite the considerable amount of research in this area there is, as yet no generally agreed mechanism linking the observed expansion with processes occ
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